OptimisticTransactionDBOptions support for C/FFI API

[vardaro]

I am trying to use rocksdb::OptimisticTransactionDB over FFI and noticed the option I need, occ_lock_buckets, isn't settable. I noticed this related issue #11703 was marked up for grabs and I went ahead and introduced OptimisticTransactionDBOptions to the C API.

Unfortunately, this contains a breaking change in the C API, in that I decided to include rocksdb_optimistictransactiondb_options_t * in the signature of rocksdb_optimistictransactiondb_open() and rocksdb_optimistictransactiondb_open_column_families().

We could preserve backwards compatibility by adding logical overloads that include the options pointer, but I am a bit partial to keeping the APIs consistent with transactiondb if possible. Happy to go the other way if the community feels otherwise. For the open functions, setting rocksdb_optimistictransactiondb_options_t * to null causes the defaults of OptimisticTransactionDB to get applied. Passing an options pointer without any of the fields mutated will assume the defaults in OptimisticTransactionDB.

I added a new overload for OptimisticTransactionDB::Open() that imitates the "simple" overload signature. It omits the CF-handle related arguments, but allows the caller to pass OptimisticTransactionDBOptions. The original "simple" overload now calls this new overload, assuming the default constructed OptimisticTransactionDBOptions. This allowed me to reduce CF-handle related logic in the C interface, and centralize more in the core RocksDB layer. This Open() refactor could stand to be its own PR, since it's useful when using the C++ interface.

[vardaro]

@pdillinger Hey Peter (or whoever might be the best person to provide feedback), can you give this a review? Thanks